This invention pertains to improving the efficiency and sensitivity of high-field high-resolution (HR) NMR techniques where an NMR lock signal for field stabilization is beneficial, especially for double-, triple-, and quad-resonance and for single-resonance quadrature techniques.
An rf lock channel tuned to a non-interfering nuclide, typically deuterium, has been routinely used to stabilize the magnetic field for at least three decades. See, for example, U.S. Pat. No. 4,110,681 by Hofer et al. Various aspects of NMR probe design are reviewed by Doty in "Probe Design and Construction" in The Encyclopedia of Nuclear Magnetic Resonance, Wiley Press, 1996.
Two basic lock approaches have been used--external and internal. In the external lock, an rf coil is wrapped around a separate, sealed capsule containing a lock sample, such as D.sub.2 O, and single-tuned to the lock frequency. For internal lock, a deuterated solvent is used for the sample solvent, and a second orthogonal coil is added around the sample coil, or one of the sample coils is multiplely tuned to include the lock frequency. In most cases, an internal lock is preferred, as this provides improved field stability and may simplify field shimming.
A typical prior art internal-lock circuit (in which one saddle coil generating transverse B.sub.1 is double-tuned to .sup.1 H and .sup.2 H) is illustrated by Doty in FIG. 2 of U.S. Pat. No. 5,162,739. A second, orthogonal saddle coil may be used to generate a second transverse B.sub.2 for another nuclide of interest. (Note that NMR saddle coils are often called "Helmholtz" coils as a carry-over from probes for iron-core magnets, although homogeneous saddle coils are more properly called "Ginsberg" coils.) This approach generally works very well for single-resonance and double-resonance NMR, as it is usually possible to achieve over 70% efficiency on the one or two nuclides (e.g., .sup.1 H, .sup.13 C) of main interest at the expense of obtaining 10% to 20% efficiency on the lock channel, which is usually quite sufficient. All prior-art lock coils for use in HR NMR, whether internal or external, are dipolar and have substantially homogeneous transverse rf magnetic field throughout the sample.
The deficiencies of the prior art become pronounced in multi-nuclear (broad-band tunable) triple-resonance NMR where high-efficiency is needed at three frequencies simultaneously. For multi-nuclear observations at high field, and especially with larger samples where balancing is required, it becomes very difficult to achieve efficiency above 50% in a double-tuned coil at one multi-nuclear resonance even when the other resonance (of the same coil) is permitted to have very low efficiency. Moreover, it is virtually impossible (always impractical) for the multi-nuclear range of a double-tuned coil to include .sup.31 P and .sup.15 N when the other resonance of this coil is tuned to .sup.2 H, as deuterium lies within the range needed for the multinuclear channel.
A substantial simplification in tuning and perhaps a factor of two improvement in efficiency on one of the channels could be obtained if internal .sup.2 H lock or homonuclear decoupling could be accomplished on a third coil having zero mutual inductance and balanced electric coupling with the other two orthogonal rf saddle coils. The obvious candidate, a solenoid aligned with the B.sub.0 axis, is not suitable, as its rf magnetic field B.sub.3 is predominately aligned with the B.sub.0 axis and is thus not capable of driving the NMR resonance. The approach taken by Anderson in U.S. Pat. No. 3,771,055 to achieve three orthogonal rf-decoupled fields works only in the transverse field geometry of the obsolete electromagnet.
Homo-nuclear decoupling, in which the sample is irradiated at the same frequency and simultaneous with signal reception, has found a few applications over the past three decades, and a recently described technique for suppression of satellites from bulk dipolar effects in concentrated liquid samples in high field 2D-NMR is likely to make homo-nuclear proton decoupling extremely important in future biomolecular NMR applications. (See P. Broekaert et al in J. Magn. Reson. Ser. A, 1996, 119, pp. 115-119.) A single-coil pulse-train technique in which the sampling occurs during windows between the pulses has been shown to be effective for some situations, but a cw technique is likely to be more effective for many applications. The problem with homo-nuclear cw decoupling is isolation between the transmitter and the receiver--a problem that was more widely appreciated before the advent of FT-NMR.
The traditional approach to cw homonuclear decoupling has been the use of two orthogonal, precisely balanced dipolar coils tuned to the proton resonance, but this approach does not work well in double- or triple-resonance multinuclear NMR as the balance requirements are extremely critical--isolation better than 40 dB is desired for two coils tuned to the same frequency.
Phased-arrays for NMR reception at a single frequency were disclosed by Carlson in U.S. Pat. No. 4,857,846 and later by others. A relatively large number of coils, each capable of generating B.sub.1 that is transverse to B.sub.0, are independently tuned to the same frequency and the signals are added with the proper phase. The tuning is simplified (and localized signal to noise may be improved) when the adjacent coils have zero mutual inductance. Various coil arrangements have been used that meet the requirements of efficiently generating substantial transverse rf magnetic field throughout a portion of the sample and having zero mutual inductance with adjacent, aligned coils. An excellent review is provided by James Hyde in `Surface Coils and Other Local Coils for In Vivo Studies`, in The Encyclopedia of Nuclear Magnetic Resonance, Wiley Press, 1996.
One requirement of coils suitable for phased arrays and low mutual inductance is that their fields must be highly non-uniform, which is completely contrary to the requirements of most HR NMR rf coils. Not surprisingly, phased arrays have not been used in HR NMR spectroscopy, although homo-nuclear rf gradient coils, as disclosed by Cory et al in U.S. Pat. No. 5,323,113, have found some applications in coherence selection, rf imaging, and solvent suppression.
Since very small magnetization nutations are effective in a lock circuit, the benefits of an internal lock are not compromised by the use of a highly non-uniform rf lock field. Thus, it is possible to design an independent lock coil that is magnetically orthogonal to two mutually orthogonal uniform rf fields but still generates substantial transverse magnetization within the sample.
With the inventive lock coils, a double-tuned multi-nuclear rf coil may be replaced by a single-tuned multi-nuclear rf coil and a separate, rf-decoupled lock coil. The rf efficiency of the multi-nuclear channel is then substantially improved--often by a factor exceeding 70%.
The various B.sub.0 shim and gradient coils described by Golay and Rumson in U.S. Pat. No. 3,569,823, Schenck et al in U.S. Pat. No. 4,646,024, and others are designed to produce orthogonal gradients in B.sub.Z, but they also generate orthogonal gradients in a transverse field (B.sub.X or B.sub.Y), although modifications could improve efficiency for transverse fields.
The homo-nuclear switchable coil used by Cory et al is capable of being switched from a homogeneous Ginsberg coil to a gradient Ginsberg coil, as described by Ginsberg and Melchner in Rev. Sci. Instrum., 41, pp. 122-123, 1970 and later by J. Friedrich and R. Freeman in J. Magn. Reson. 77, pp. 101-118, 1988. This is predominately a dB.sub.x /dx-dB.sub.Y /dy coil and this gradient field is mathematically orthogonal to the transverse dipolar rf field when integrated over the sample region, as required for coherence rejection when the coil configuration is switched between excitation and reception. The two configurations are also orthogonal when integrated over all space and hence have zero mutual inductance, although the rf filling factor of this gradient coil is rather low. It should be noted that Friedrich and Freeman use the rf gradient coil for pre-saturation to permit localized spectroscopy using one half of the coil following the presaturation sequence. Cory's invention may be thought of as imparting a phase dependence to specific coherences throughout the sample in such a way that their signals average to zero when received by an orthogonal coil.
Various fields and coils are often described loosely as being orthogonal, but in most real cases the orthogonality condition (the integral of the product of the functions over a given region is zero) applies only over a very limited region of space--typically the sample region, or a fraction thereof, for NMR shim coils. When two coils are mathematically orthogonal over all space, they have zero mutual inductance L.sub.M. Expressed otherwise, their inductive coupling coefficient k is zero, which is defined by the following: EQU L.sub.M =k.sqroot.(L.sub.1 L.sub.2)
where L.sub.1 and L.sub.2 are the self inductances of the respective coils. "Orthogonal" shim coils may have coupling coefficients above 0.4, as they are normally used only for DC field corrections in the sample space, while MRI gradient coils generally have coupling coefficients below 0.02.
The main differences between conventional shim or gradient coils and the inventive non-dipolar rf coils in optimization criteria are (1) the rf coil inductance should be about three orders of magnitude lower, (2) the filling factor and Q of the rf coil should be maximized, (3) no particular functional dependence of the field profile in the sample region is required, and (4) the orthogonality integration must extend over all space rather than just the sample region to achieve zero mutual inductance.